An upper-level shortwave produced thunderstorms across north-central Wisconsin during the evening hours. Three tornadoes and wind damage was reported as these thunderstorms tracked through the area.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS GRB wrote it up.
An upper-level shortwave produced thunderstorms across north-central Wisconsin during the evening hours. Three tornadoes and wind damage was reported as these thunderstorms tracked through the area.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Green Bay, WI wrote its last area forecast discussion before the first event at Aug 9, 6:24 PM CST, 20 minutes earlier.
Area Forecast Discussion National Weather Service Green Bay WI 724 PM CDT Sun Aug 9 2020 Updated aviation portion for 00Z TAF issuance .SYNOPSIS... Issued at 330 PM CDT Sun Aug 9 2020 A round or two of thunderstorms tonight into Monday, then turning much less humid but with above normal temperatures continuing throughout the upcoming work week. The large scale pattern across North America won't change much during the next 7 days. The subtropical ridge will remain stretched out across the central and southern CONUS, with a seasonably strong band of westerlies to the north across Canada and the far northern CONUS. Within the westerlies, a ridge position was off the West Coast, with a trough position over the Intermountain West. Those features will remain in place for a few days, then the pattern will become slowly progressive. The humidity will drop significantly early in the period, then slowly creep back upward thereafter. High temperatures should remain slightly to modestly above normal throughout the period. The dry air may allow low temperatures to drop AOB normal for a few days during the early to middle part of the period, otherwise mins will probably be at least slightly above normal. The pattern will probably result in AOA normal precipitation, most of which will fall early in the period. A several day long stretch of dry weather is likely during the upcoming work week, then rain chances will gradually begin to increase. && .SHORT TERM...Tonight and Monday Issued at 330 PM CDT Sun Aug 9 2020 A quiet early afternoon ongoing across the area, as the area is situated in the warm sector, with a mix of low, mid and upper level clouds spreading across the state. It was warm and humid, with temps climbing into the 80s and dewpoints in the upper 60s and lower 70s, producing heat index readings in the middle 80s and lower 90s. As expected, a very messy upstream pattern with multi areas of storms ongoing through the late morning and early afternoon. Forecast challenge will be trying to pin down where/when the best chances for showers/storms will occur and the prospects for severe weather. For tonight...CAMs continue to struggle with the complicated set up. Currently there are clusters of storms across northeast IA/southern WI, eastern MN, northwest WI and eastern SD and far northwest MN. Each one of these will either bring a chance of storm activity to the area or impact the chances of that happening. First, the storms across far northeast IA and southwest WI continue to hold its own, despite the lack of bulk shear, moving east/eastnortheast. This will continue to track toward mainly east central WI, but the models that do show this feature have it weakening. Second, the well defined MCV that has been tracking across central MN. This feature has been behaving so far, fighting some subsidence and weak capping, but it is moving into a more favorable environment, so will likely see an uptick in activity in the late afternoon/evening as it approaches. This will track into the Northwoods, bringing the best chance for widespread activity through the early evening hours. If the system becomes more mature, activity along the southern edge could expand, impacting more of the area, but it will be fighting the cap. Finally, the convection across eastern SD and far northwest MN associated with the actual cold front and shortwave. This has the most organized severe weather threat with the best dynamics with several ongoing warnings. This activity will continue to drop southeast through the night, but again, models differ on how long it will last as it enters a more stable/worked over airmass across northeast MN where there is ongoing convection. The boundary/outflow from this feature could help kick off additional storms later tonight, as it reaches northwest WI where more instability should be present. Model soundings still showing a cap in place across the area, so expect any activity outside the more organized areas will struggle to develop/sustain itself. Latest runs of the HRRR is one of the better performing models, so will lean on its guidance, but keep chance POPs going for most locations for much of the night as confidence remains on the low side on how everything will play out tonight. Looking at the severe weather threat, SBCAPE of 2500-3500 J/kg were found across the area this afternoon, despite the cloud cover for much of the day. Low-mid level lapse rates continues to steepen to between 7-9C/km. Bulk shear the greatest (25-35 kts) across north central WI, with values falling to around 15 kts across east central WI. So atmosphere conducive for severe weather, if we can get any area of organized convection. Damaging winds looks to be the main threat, with hail possible as well. The other hazard will be heavy rain and flooding as PWATs remain between 1.5-2.0". It will be another warm and muggy night across the area, with lows dropping into the middle 60s north to the lower 70s in the Fox Valley and lake shore and dewpoints remaining in the upper 60s to around 70. For Monday...surface cold front and shortwave sweep across the area, bringing additional chances for showers/storms. Models again not in the best agreement due to not handling the overnight activity very well. There could be some left over activity across a portion of the area to start the day, then focus will be along the front as it passes through the area, reaching the Fox Valley and lake shore in the late morning or early afternoon. If the front slows at all, chances for a stronger round of storms increase, along with possibly a severe storm. Models that do not show much activity tonight favor a higher end event on Monday (HRRR showing 3500+ CAPE and 30 kts of bulk shear for the front to work with). So this will need to be watched. Locally heavy rain and flooding will be possible with PWATs of nearly 2" pooling ahead of the front. Warm and humid conditions will continue in the morning, then the drier air will filter in behind the front, but won't reach the Fox Valley and lake shore until evening. .LONG TERM...Monday Night Through Sunday Issued at 330 PM CDT Sun Aug 9 2020 Quiet, dry weather is likely for at least a few days at the start of the long-term portion of the forecast as a weak anticyclone slowly shifts across the region. Westerly flow aloft will limit the cooling associated with the high, but the drop in humidity will be much more noticeably. Overall, saw no reason to deviate significantly from the standard forecast initialization grids generated from a broad-based blend of guidance products. && .AVIATION...for 00Z TAF Issuance Issued at 723 PM CDT Sun Aug 9 2020 More widespread thunderstorm activity that was anticipated this evening, thus far not panning out due to more clouds than expected across parts of the area today and elevated warm layer that has generally capped the environment. Through at least mid night or so, appears best chance for any scattered thunderstorm activity will be confined across the far northern Wisconsin in vicinity of convectively induced circulation. A few of these storms storms may contain gusts to 40 kts and brief MVFR cigs and vsbys. Appears the brunt of this activity will remain NNW of RHI TAF site through late evening. Later tonight, primary frontal boundary over NW Minnesota will continue to push southeast into far northwest Wisconsin after midnight. Latest short-term guidance still suggests that some scattered thunderstorm activity may initiate along this front and perhap impact AUW/CWA and RHI sometime in the 09 to 12 UTC time frame. However, given poor performance of the guidance lately and elevated warm layer aloft, confidence is low on location and timing. Elsewhere overnight, copius moisture may lead to low cloud formation and fog across parts of central and northcentral Wisconsin as well. This may bring some areas of IFR cigs and vsbys across this area. Frontal boundary will push east across the entire forecast area during the day on Monday and may kick off a few more showers and thunderstorms prior to 21 UTC especially across the Fox Valley. && .GRB WATCHES/WARNINGS/ADVISORIES... NONE. && SYNOPSIS.......Skowronski SHORT TERM.....Bersch LONG TERM......Skowronski AVIATION.......ESB
Hazardous Weather Outlook National Weather Service Green Bay WI 324 PM CDT Sun Aug 9 2020 WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-102030- Vilas-Oneida-Forest-Florence-Northern Marinette County-Lincoln- Langlade-Menominee-Northern Oconto County-Door-Marathon-Shawano- Wood-Portage-Waupaca-Outagamie-Brown-Kewaunee-Waushara-Winnebago- Calumet-Manitowoc-Southern Marinette County- Southern Oconto County- 324 PM CDT Sun Aug 9 2020 This Hazardous Weather Outlook is for north-central and northeast Wisconsin. .DAY ONE...Tonight Isolated to scattered thunderstorms are expected this evening and tonight. Some of the thunderstorms could be severe with damaging winds and large hail. The storm will also produce torrential rainfall which could lead to localized flooding. .DAYS TWO THROUGH SEVEN...Monday through Sunday Thunderstorms may redevelop Monday, especially in east-central Wisconsin. A few of the storms could be strong and produce small hail, gusty winds and heavy rainfall. There is a chance of thunderstorms will return late in the work week and continue into next weekend. It is too early to determine if any of the storms will become severe. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed tonight. Spotters are requested to report any wind damage, large hail, flooding or rainfall over an inch to the National Weather Service in Green Bay. LMZ521-522-541>543-102030- The Bay of Green Bay south of a line from Cedar River MI to Rock Island Passage-Lake Michigan from Washington Island to Sheboygan- 324 PM CDT Sun Aug 9 2020 This Hazardous Weather Outlook is for portions of northwest Lake Michigan and the waters of Green Bay. .DAY ONE...Tonight Thunderstorms may pose a hazard to boaters on the bay and lake tonight. Some of the storms could produce strong winds and large hail. .DAYS TWO THROUGH SEVEN...Monday through Sunday Thunderstorms may still pose a hazard to boaters again on Monday. Bersch/Skowronski
Special Weather Statement
National Weather Service Green Bay WI
711 PM CDT Sun Aug 9 2020
WIZ005-010-011-100115-
Vilas-Oneida-Forest-
711 PM CDT Sun Aug 9 2020
...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT VILAS...NORTHWESTERN
FOREST...AND NORTHERN ONEIDA COUNTIES...
At 710 PM CDT, strong thunderstorms were clustered near Lac Du
Flambeau, or 19 miles east of Turtle Flambeau Flowage, moving
northeast at 35 mph.
Wind gusts up to 50 mph and pea size hail are possible with these
storms.
Locations impacted include...
Eagle River, Lac Du Flambeau, Phelps, Boulder Junction, Winchester,
Headwaters Wilderness, Kentuck Lake Campground, Border Lakes Natural
Area, Squirrel Lake and Johnson Lake Barrens Natural Area.
Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.
Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
LAT...LON 4617 8923 4602 8882 4603 8879 4600 8868
4598 8867 4593 8867 4572 8905 4572 9004
4598 9005 4598 8993 4630 8993
TIME...MOT...LOC 0010Z 236DEG 29KT 4598 8980
EBThe Marquette radar (KMQT), 158 km away, was scanning. The volume nearest the first event began at 00:43 UTC.
4 tornado, 2 thunderstorm wind. Local standard time, as filed.
A tornado produced a path of tree damage (DI 27,28, DOD 3,4) from the eastern shore of Star Lake to the Upper Bucktabon Springs State Natural Area. Peak winds were estimated around 95 mph.
Thunderstorm winds downed numerous trees near Land O' Lakes. The time of this report is an estimate based on radar data.
A tornado produced tree damage (DI 27,28, DOD 3,4) from east of St. Germain to southwest of Eagle River with intermittent touchdowns along the Wisconsin River. Peak winds were estimated around 90 mph.
A tornado formed at 9:12 PM CDT in a remote forested area in extreme northeast Forest County, east of the town of Alvin. The tornado moved northeast to about 0.3 miles north of Ransdell Lake, then turned east, reaching maximum width before crossing Huff Creek Road. The tornado produced a path of tree damage (DI 27,28, DOD 3,4) before entering Florence County at 9:18 PM CDT. Peak winds were estimated at 105 mph.
The tornado that formed at 9:12 PM CDT in extreme northeast Forest County crossed the county line at 9:18 PM CDT. The tornado continued east, producing a path of tree damage (DI 27,28, DOD 3,4) in a remote forested area in extreme northwest Florence County, before crossing the Brule River into Upper Michigan at 9:20 PM CDT. Peak winds were estimated at 105 mph.
Thunderstorm winds downed trees and power poles in Long Lake. The time of this report is an estimate based on radar data.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wisconsin in 2020Every state in 2020This day in weather historyThe tornado archive for Wisconsin, 2020Warnings in Wisconsin, August 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 902713, 902715, 911932, 908343, 908351, 903095.